Energy harvesting (also known as power harvesting or energy scavenging) is a process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), captured, converted into electrical energy and stored for low-power wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. For example, piezoelectric transducers are employed for harvesting electrical power from vibrations. Different AC-DC converters are described in the literature in order to rectify the AC power and extract the maximum amount of power.
Possible applications of energy harvesters comprising such AC-DC converters for piezoelectric generators are, e.g., in applications like highway bridges (structural health monitoring) or railway trains (tracking and tracing). The frequency range of the vibrations associated to these applications is between 2 and 50 Hz, whereas mean accelerations are around 0.1 g.
The admittance locus of a piezoelectric transducer has intrinsic information about for which piezoelectric transducers the employment of a SSHI converter instead of a diode bridge will provide a significant increase in the harvested power. The internal impedance of a piezoelectric element is complex, as described by J. Brufau-Penella and M. Puig-Vidal in “Piezoelectric energy harvesting improvement with complex conjugate impedance matching,” Journal of Intelligent Material Systems and Structures, vol. 00-2008, 2008. Therefore, the maximum output power of the piezoelectric. transducer is obtained when the complex conjugate of the internal impedance is connected as output load. However, this solution is unrealistic since the inductance needed as complex conjugate load would be too large due to the dominant capacitive characteristic of piezoelectric elements. If a resistor is connected as a load to the piezoelectric element, the output power obtained depends on the mechanical frequency that excites the piezoelectric element and the resistance. The maximum output power in this case is typically obtained with a resistance that is equal to the modulus of the equivalent Thevenin impedance of the piezoelectric element at the frequency where the ratio of the real and imaginary parts of the admittance of the piezoelectric element is maximized. Thus, the maximum output power is obtained at the frequency where the admittance of the piezoelectric element has its most resistive behavior.
The peak value of the ratio between the output power when a resistor is the load and the maximum power, depends on the impedance circle of the piezoelectric element. There are piezoelectric elements were the maximum of this ratio is close to one and others were this ratio is much lower. For the piezoelectric elements with a ratio close to 1, the employment of non-linear techniques is not going to provide an improvement over the rectifier bridge. However, for piezoelectric elements where this ratio is far away from 1, the non-linear converters are a better solution than the rectifier bridge.
The Synchronized Switch Harvesting on Inductor (SSHI) is a non-linear switching technique that provides DC (direct current) power from an energy source, such as a mechanically excited piezoelectric element. A SSHI converter typically consists of a switch and an inductor plus a diode bridge. The mechanically excited piezoelectric element typically produces an alternating electric voltage and an alternating current (AC).
An efficient AC-DC converter for piezoelectric elements during vibration is of special importance for maximizing the harvested power.
The AC power delivered by, for example, piezoelectric transducers can be rectified with a diode bridge and a filtering capacitor (linear technique).
A more recent AC-DC converter which employs an inductor connected through a switch to the piezoelectric element (called SSHI) is also available. The switch is closed when the piezoelectric peak displacement is reached. The connection of the piezoelectric element with the inductor causes a resonant effect and a fast inversion of the piezoelectric voltage. After the piezoelectric voltage inversion, the switch is opened until a new peak is detected. However, the diodes of the diode bridge still induce voltage gaps which causes losses in the harvesting circuit and limits the harvested power.
In a further development of AC-DC converters for SSHI converters, the switch or switches for the inversion of the piezoelectric voltage are separated and two diodes of the diode bridge are replaced by these switches. Hence, the circuit includes less components, thus reducing the cost and the dimensions of the circuit. Another benefit is the removal of the diodes that induce voltage gaps. Therefore, losses introduced by such voltage gaps in the harvesting circuit are limited and the harvested power is thus greater. However, the amount of energy extracted from the piezoelectric element per cycle may still be comparatively low and only a fraction of theoretically extractable energy amount.